Among flat panel display (FPD) devices, a liquid crystal display (LCD) device has advantages in visibility, power consumption and heat emission as compared with a cathode ray tube (CRT). Accordingly, the LCD device has been the subject of recent research and development with a plasma display panel (PDP) and an electroluminescent display (ELD) as a next generation display unit for a mobile phone, a monitor of computer and a television.
LCD devices make use of optical anisotropy and polarization properties of liquid crystal molecules to produce images. Specifically, since the liquid crystal molecules have a definite alignment as a result of their long, thin shapes and are arranged to have initial pre-tilt angles, an alignment direction of the liquid crystal molecules can be controlled by applying an electric field. Accordingly, variations in the applied electric field influence the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling the applied electric field, an image that has a desired brightness can be produced.
Since an LCD device is a non-emissive type display device, an additional light source is required. Accordingly, a liquid crystal display module of an LCD device includes a liquid crystal display panel and a backlight unit disposed under the liquid crystal display panel. Light from the backlight unit is supplied to the liquid crystal display panel and images are displayed by adjusting transmittance of the light according to arrangement of liquid crystal molecules.
FIG. 1 is an exploded perspective view showing a liquid crystal display module according to the related art. In FIG. 1, a liquid crystal display module 5 includes a bottom frame 15, a main frame 40, a top frame 70, a liquid crystal display panel 45 and a backlight unit 10. The liquid crystal display panel 45 and the backlight unit 10 are surrounded by the bottom frame 15, the main frame 40 and the top frame 70. The backlight unit 10 disposed under the liquid crystal display panel 45 includes a reflecting plate 30, a light guide plate 25 over the reflecting plate 30, a plurality of optic films 35 over the light guide plate 25 and a light emitting diode (LED) array 60 at a side of the light guide plate 25. The backlight unit 10 including the LED array 60 may be referred to as an LED backlight unit.
The backlight unit 10 is disposed on the bottom frame 15 and one side of the bottom frame 15 may be used as a guide panel 75 where the LED array 60 is fixed. The guide panel 75 has a bent surface to cover the plurality of optic films 35.
FIG. 2A is a cross-sectional view showing a liquid crystal display module having an LED backlight unit according to the related art, and FIG. 2B is a perspective view showing an LED backlight unit according to the related art. In FIGS. 2A and 2B, an LED backlight unit includes a reflecting plate 30, a light guide plate 25, a plurality of optic films 35, an LED array 60 and a guide panel 75. The light guide plate 25 is disposed over the reflecting plate 30 and the plurality of optic films 35 is disposed over the light guide plate 25. In addition, the LED array 60 is disposed at a side of the light guide plate 25. The guide panel 75 where the LED array 60 is fixed has a bent surface to cover the plurality of optic films 35. A liquid crystal display panel 45 is disposed over the plurality of optic films 35. A bottom frame 15 wraps the reflecting plate 30 and is combined with the guide panel 75.
The LED array 60 is spaced apart from the light guide plate 25 to minimize an influence of heat from the LED array 60 on the plurality of optic films 35 and prevent a hot spot of the liquid crystal display panel 45. The LED array 60 includes a plurality of LED lamps 60a and an LED printed circuit board (PCB) 60b. The plurality of LED lamps 60a are fixed on the LED PCB 60b and the LED PCB 60b may include a metallic material. The reflecting plate 30 is disposed under the light guide plate 25 and the LED array 60. Light downwardly scattered at the light guide plate 25 is induced to re-enter the light guide plate 25 by the reflecting plate 30. In addition, the reflecting plate 30 extends under the LED array 60.
When an electric power is supplied to the liquid crystal display module, a voltage is applied to the LED array 60 by an inverter (not shown) to emit light and the light enters the light guide plate 25. The light emitted from the guide plate 25 passes through the plurality of optic films 35 and the liquid crystal display panel 45.
However, since the light guide plate 25 and the LED array 60 are spaced apart from each other, the light from the LED array 60 passes through an upper portion of a space between the light guide plate 25 and the LED array 60 to be absorbed by the guide panel 75 or to be scattered outside the plurality of optic films 35. As a result, light efficiency of the LED backlight unit is reduced.
FIG. 3 is a graph showing light efficiency and light loss of an LED backlight unit according to the related art. In FIG. 3, a horizontal axis represents a gap distance between a light guide plate and an LED array, and a vertical axis represents a light efficiency and a light loss. As the gap distance increases, the light efficiency decreases and the light loss increases such that the light efficiency is in inverse proportion to the light loss. Accordingly, the light efficiency and the light loss are further improved as the gate distance between the light guide plate and the LED array. For example, when the gap distance is about 0 mm, i.e., the light guide plate contacts the LED array, the light loss is about 1.9%.
However, when the gap distance is about 0 mm, the light guide plate may be deteriorated due to heat from the LED array. For example, the light guide plate may be colorized with yellow or may be deformed. In addition, light passing through the light guide plate may have deterioration in uniformity such as a stripe pattern due to straightness of light from the LED array.
Further, when the light guide plate is spaced apart from the LED array to prevent above disadvantages, light efficiency is reduced.